Systems and methods for cable equalization

ABSTRACT

Provided herein are methods and systems that provide automatic compensation for frequency attenuation of a video signal transmitted over a cable. In accordance with an embodiment, a system includes an equalizer and a compensation controller. The equalizer receives a video signal that was transmitted over a cable, provides compensation for frequency attenuation that occurred during the transmission over the cable, and outputs a compensated video signal. The compensation controller automatically adjusts the compensation provided by the equalizer based on comparisons of one or more portions of the compensated video signal to one or more reference voltage levels.

PRIORITY CLAIM

This application claims priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application No. 61/110,917, entitled “System andMethod for Cable Equalization” (Attorney Docket No. ELAN-01178US0),filed Nov. 3, 2008, which is incorporated herein by reference.

BACKGROUND

Category 5 (CAT5) is an Ethernet cable standard defined by theElectronic Industries Association and Telecommunications IndustryAssociation (EIA/TIA). CAT5 cable, which includes four unshieldedtwisted-pairs of wire, was originally intended to support Fast (100Mbps) Ethernet and comparable alternatives such as Asynchronous TransferMode (ATM). As with all other types of twisted pair EIA/TIA cabling,CAT5 cable runs are intended to be limited to a maximum recommended runrate of 100 m (328 feet).

The ubiquity and cost-effectiveness of CAT5 cabling make it anattractive choice for video distribution. Hotels and office buildingsare perfect environments for video distribution, and are often alreadywired with CAT5 unshielded twisted-pair (UTP) cable. CAT5 is alsocheaper and easier to install than coaxial cable. However, CAT5 cablewas originally not intended for high bandwidth video signalapplications, since it has substantial attenuation as frequenciesincrease. In video applications, high frequencies are used to representboth sharp image details and color information. Thus, this attenuationcan seriously impact picture quality. Accordingly, there is a need toovercome the high frequency attenuation that occurs when using CAT5cable, or similar cable, for video signal transmission.

Since unshielded twisted-pair (UTP) cables are now being used for videotransmission, companies have begun to design receivers and equalizersthat specifically compensate for the high frequency attenuation causedby such cables. One example of this is the EL9110 DifferentialReceiver/Equalizer available from Intersil Corporation, of Milpitas,Calif. This device accepts a control voltage signal that can be used toset the compensation levels required for different lengths of cable.Thus, if a specific receiver/equalizer is always receiving a videotransmission over the same cable of unchanging length, the compensationlevel at the receiver/equalizer can be manually set once, and videosignals should be correctly compensated thereafter. However, a challengeexists where a receiver/equalizer can receive video transmission fromvarious different transmitters, over cables of various differentlengths, such as may occur in a building that is wired for videoconferencing. In such a case, each time a receiver receives a videotransmission over a cable of a different length, the compensation levelneeds to be adjusted. It would be beneficial if systems and methods wereavailable to perform such adjustments. It would also be beneficial ifsuch adjustments could be automatic.

SUMMARY

Embodiments of the present invention relate to systems and methods thatprovide automatic compensation for frequency attenuation of a videosignal transmitted over a cable. In accordance with an embodiment, sucha system includes an equalizer and a compensation controller. Theequalizer receives a video signal that was transmitted over a cable,provides compensation for frequency attenuation that occurred during thetransmission over the cable, and outputs a compensated video signal. Thecompensation controller automatically adjusts the compensation providedby the equalizer based on comparisons of one or more portions of thecompensated video signal to one or more reference voltage levels.

In an embodiment, the equalizer includes a high band equalizer thatcompensates for high frequency attenuation caused by the cable, a lowband equalizer that compensates for low frequency attenuation caused bythe cable, and a DC gain controller that fine tunes DC gain of theequalizer so that an average level of sync tips, of horizontal syncpulses within the compensated video signal, is substantially equal to apredetermined nominal level.

Certain embodiments of the present invention are directed to a high bandequalizer that includes a plurality N of equalizer stages connected inseries, each including a differential input and a differential output.In an embodiment, a 1^(st) one of equalizer stages is optimized for a1^(st) length of the cable, a 2^(nd) one of said equalizer stages isoptimized for a 2^(nd) length of the cable, and an N^(th) one of saidequalizer stages is optimized for an N^(th) length of the cable, where Nis equal to or greater than 3. The high band equalizer also includes afirst selector, a second selector and a weighted averager. The firstselector has N inputs connected to the inputs of each of the N equalizerstages, and an output. The second selector has N inputs connected to theoutputs of each of the N equalizer stages, and an output. The weightedaverager has inputs connected to the outputs of the first and secondselectors, and has an output. The first and second selectors are used toselect which equalizer stages are active and which equalizer stages areinactive. The weighted averager produces at its output, a weightedaverage of the signal input to and output from the last active equalizerstage. A compensation controller can control the first and secondselectors and the weighted averager.

This summary is not intended to summarize all of the embodiments of thepresent invention. Further and alternative embodiments, and thefeatures, aspects, and advantages of the embodiments of invention willbecome more apparent from the detailed description set forth below, thedrawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a high level block diagram of an equalizer, according to anembodiment of the present invention, that provides for automaticcompensation of a video signal received through a length of cable (e.g.,up to a mile, or even longer) of arbitrary length.

FIG. 1B is a high level block diagram of an equalizer, according to anembodiment of the present invention, that provides for manualcompensation of a video signal received through a length of cable (e.g.,up to a mile, or even longer) of arbitrary length.

FIG. 2A is a block diagram that shows details of the high band equalizerblock of FIGS. 1A and 1B, according to an embodiment of the presentinvention.

FIG. 2B is a diagram that provides exemplary details of the equalizerstages of FIG. 2A, according to an embodiment of the present invention.

FIG. 3 is a diagram that provides exemplary details of the low bandequalizer of FIGS. 1A and 1B, according to an embodiment of the presentinvention.

FIG. 4 is a block diagram that shows details of the level sense circuitand the level detect to digital circuit of FIGS. 1A and 1B, according toan embodiment of the present invention.

FIGS. 5A & 5B are high level block diagrams of systems that includes theequalizer of FIG. 1A or 1B.

FIG. 6 is a high level flow diagram that is used to summarize variousmethods of embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1A is a high level block diagram of an equalizer 100, according toan embodiment of the present invention, which can be used to recover asignal received through a long length of cable (e.g., up to a mile, oreven longer), where the cable can be any arbitrary length. The cablecould be a CAT5 cable or a CAT6 cable, but is not limited thereto. Sucha signal is sent differentially across two wires of such cable, e.g., atwisted pair of wires, in the case of a CAT5 or CAT6 cable. The receivedsignal can be, e.g., a NTSC video signal, or some other type of videosignal. The equalizer 100 compensates for the high frequencyattenuation, and to a lesser extent low frequency attenuation, caused bythe cable and, and the equalizer 100 outputs a compensated video signal116. The frequency compensation provided by the equalizer 100 should beappropriate to the length of the cable and the type of the cable thatcaused the frequency attenuation. The compensated video signal 116, onceproperly compensated, is substantially equivalent in frequency and phasecontent to the video signal before its content was altered by the cable.In the FIG. 1A and the other FIGS., “SE” stands for “single ended” and“Diff.” stands for “differential”.

Referring to FIG. 1A, the equalizer 100 is shown as including adifferential input buffer 102, a differential high band equalizer 104, adifferential to single ended converter 106, a low band equalizer 108, aDC gain control circuit 110, a low pass filter 112 and an output driver114. Additionally, the equalizer 100 is shown as including an automaticcompensation controller 118, which automatically adjusts a highfrequency compensation control signal 146, a low frequency compensationcontrol signal 147, and a DC gain control signal 148, so that thecompensated video signal 116 output by the equalizer 100 is correctlycompensated. More generally, the automatic compensation controller 118controls the high band equalizer 104, the low band equalizer 108 and theDC gain control circuit 110.

Note that there is some flexibility in the order of the signalprocessing blocks. For example, the low pass filter 112, DC gain and lowband equalizer 108 can occur anywhere after the high band equalizer 104.However, due to noise considerations, the high band equalizer 104 shouldeither be at the input, or immediately following the input buffer 102.The video signal only needs to become single-ended by the time itreaches the output, so the differential to single ended converter 106can be anywhere between the input and the output driver 114. However,concerns regarding noise and supply isolation lead to the preferencethat the high band equalizer 104 be implemented in the differentialmode. It is also possible that further processing blocks be included,such as, but not limited to, a group delay equalizer that is used toensure color fidelity.

The video signal 116 output by the equalizer 100 is provided to both ahorizontal sync (H-sync) detector 132, and a level sense detector 142.The H-sync detector 132 outputs a horizontal sync signal 133, which isprovided to a sync pulse to digital timing circuit 134. In a well knownmanner, the circuit 134 can lock to the H-sync pulses, detect the timingof the leading and/or trailing edges of the H-sync pulses (preferablythe trailing edges) within lines of the video signal 116. Based on suchdetected timing, other portions of lines of the video signal 116 can bedetected, including, e.g., the blanking level portion and the colorburst portion (if the video signal is a color signal). Additionally, thebeginning of the next H-sync pulse can be detected. The blanking levelis the nominal voltage of a video waveform during the horizontal andvertical periods, excluding the more negative voltage sync tips. Usingthis knowledge of the various portions of the video signal 116, thelevel detect to digital circuit 144 can provide appropriate pulses forsampling the various portions of the video signal 116, and together withthe level sense detector 142 can perform selective comparisons of thevarious portions of the video signal 116 to appropriate referencevoltages, including a blanking level reference voltage, a burst levelreference voltage, a sync level reference voltage and a monochromereference voltage. In accordance with an embodiment, the monochromereference voltage is set between the blanking level reference voltageand the burst level reference voltage, and can be used to determinewhether the video signal is a color or monochrome signal. Additionaldetails of the level sense detector 142 and the level detect to digitalcircuit 144 are discussed below with reference to FIG. 4.

The differential input buffer 102 prevents the equalizer 100 fromloading and interfering with operation of the cable (e.g., cable 504 inFIG. 5A) and the transmitter (e.g., 502 in FIG. 5A) on the far end ofthe cable. Such functionality of the differential input buffer 102 canbe provided by a first stage (e.g., G0 or G1 in FIGS. 2A & 2B) of thedifferential high band equalizer 104, and thus, the input buffer 102need not be included in some configurations. This buffer 102 can alsoprovide a high input impedance which best allows a common mode (CM)clamp 120 and a differential clamp 122 to correctly drive an inputcoupling capacitor pair.

The differential high band equalizer 104, which is controlled by thehigh frequency compensation control signal 146, boosts the highfrequencies of the received differential video signal 101. Additionaldetails of the differential high band equalizer 104, according to anembodiment of the present invention, are discussed below with referenceto FIGS. 2A and 2B.

A differential to single ended converter 106 converts the differentialoutput of the high band equalizer 104 to a single ended signal, so thatfurther processing of the signal, including low band equalization andgain control, can be performed on a singled ended signal. Alternatively,the single ended converter 106 can be removed, or performed furtherdownstream, and all or some of the further processing can be performeddifferentially. In fact, the compensated video output signal 116 can bea differential signal in some applications/implementations.

The low band equalizer 108, which is controlled by the low frequencycompensation control signal 147, performs boosting of the lowfrequencies of the received video signal 101. Additional details of thelow band equalizer, according to an embodiment of the present invention,are discussed below with reference to FIG. 3.

The DC gain control 110, which is controlled by the DC gain controlsignal 148, fine tunes DC gain of the amplifier so that horizontal syncpulses within the compensated video signal 116 have a predeterminednominal level. In this manner, the DC gain control 110 can compensatefor non-standard video levels originating from the video source.

Since video signals typically have frequencies of interest from about 30Hz to about 6 MHz, the low pass filter 112 can have a cut-off frequencyof about 6.5 or 7 MHz, but is not limited thereto. In alternativeembodiments, the low pass filter 112 can be located between thedifferential to single ended converter 106 and the low band equalizer108, or between the low band equalizer 108 and the DC gain controlcircuit 110.

The output driver 114 can be, e.g., a 75 Ohm output driver, but is notlimited thereto.

Still referring to FIG. 1A, the equalizer 100 is also shown as includinga common mode (CM) clamp 120 and a differential clamp 122. The CM clamp120 samples the common mode of the first stage of the equalizer 100,which can be the differential input buffer 102, or as explained withreference to FIG. 2A, can be a first stage (e.g., 202 ₀) of thedifferential high band equalizer 104. The CM clamp 120 feeds back acommon mode current, which in conjunction with the input couplingcapacitors, (with the same current for both differential inputs)establishes the common mode level at the input. The differential modeclamp 122 senses the compensated video output signal 116 (SE Video Out)and feeds back a differential current to the input which in conjunctionwith the input coupling capacitors, (Diff. Video In) establishes thedesired differential voltage. Measurements done at the output arerelative to the reference level established by this clamp 122. Thereference level can be, e.g., ground, or for a single supply circuit canbe VDD/2, but is not limited thereto. Both clamps 120 and 122 can besampled by a same clamp pulse generated by the sync pulse to digitaltiming circuit 134. The clamp pulse can occur in the back porch area ofthe video following the sync pulse. This is the traditional black level(or more correctly, blanking level) for composite video. Stated moregenerally, the clamps 120 and 122 force a specific portion (either theback porch or the sync tip) of the video signal to a specific DCvoltage, to restore the DC level and maintain an internal operatingrange that ensures substantially linear operation of the internalcircuitry.

In accordance with an embodiment of the present invention, vertical sync(V-sync) pulses are ignored when providing automatic compensation forfrequency attenuation of a video signal transmitted over a cable. Thus,V-sync pulses should be distinguished from H-sync pulses, which can bedone in various manners. For example, it is known that V-sync pulses arelonger than H-sync pulses. Thus, V-sync pulses and H-sync pulses can bedistinguished based on their length. In a specific embodiment, V-syncpulses and H-sync pulses can be distinguished by detecting the synclevel using both the H-sync detector 132 and the level sense detector142. Once an internal timer is locked to sync pulses, the sync level canbe sensed at various points in a line, e.g., at 25% and/or 75% points inthe line (timing-wise). A counter can be reset on the first of thesepoints, and can be increment once per line (e.g., at the H-sync pulseedges). In an embodiment, the control loops used for automaticcompensation for frequency attenuation are only updated on lines thatare well past the vertical interval (e.g., lines 25 onward), and beforethe start of the next line that would include a vertical sync pulse(e.g., 262.5 lines after a previous V-sync pulse in NTSC, or 312.5 linesin PAL).

FIG. 1B is a high level block diagram of an equalizer 100′, according toanother embodiment of the present invention, that provides for manualcompensation of a video signal 101 received through a long length ofcable (e.g., up to a mile, or even longer) of arbitrary length. Theequalizer 100′ includes a manual compensation controller 119 that allowsa user to make manual compensation adjustments, e.g., using knobs,switches, sliders, buttons, a graphical user interface, or the like. Forexample, the output video signal 116 can be provided to a displaymonitor (or video waveform monitor or vectorscope, or any other type ofsuitable video test equipment), and the user can manually makeadjustments until the video image on the display monitor is optimized,or at least satisfactory, and based on the type of cable, and whetherthe signal is color or monochrome (e.g., black and white).

FIG. 2A is a high level block diagram of the high band equalizer 104,according to an embodiment of the present invention. Note that thevarious stages, the inputs and the outputs shown in FIG. 2A, aredifferential signals, but that single lines have been drawn so at to notunnecessarily clutter the figure. Referring to FIG. 2A, the high bandequalizer 104 is shown as including six equalizer stages 204 ₁-204 ₆,each of which is associated with a corresponding attenuation stage 202₁-201 ₆. The attenuation stage 202 _(n), either passes through thesignal at its input (i.e., provides no attenuation), or provides apredetermined (e.g., −6 dB) amount of attenuation, where suchattenuation can be used to prevent overload of an equalizer stage 204_(n+1). For simplicity, an attenuation stage 202 _(n) and itscorresponding equalizer stage 204 _(n) can collectively be referred tosimply as an equalizer stage 203 _(n).

In accordance with an embodiment of the present invention, eachsuccessive equalizer stage 204 provides high frequency boosting for anadditional equal length of cable (e.g., an additional 1000 feet ofcable), in the frequency range of about 500 kHz to about 6 MHz, witheach equalizer stage having a slightly different equalization curve thanthe other equalizer stages. Further, each equalizer stage 204 ₁-204 ₆can be designed to boost the signal at the upper frequency of interest(e.g., 6 MHz) by a same amount (e.g., about 12 dB) while boosting thesignal at a lower frequency of interest (e.g., 1 MHz) by a lower amount(e.g., about 5 dB), with the DC Gains between 0 dB and 1 dB.Accordingly, the total sequentially (i.e., serially) connected chain ofequalizer stages 204 ₁-204 ₆ can be capable of approximately 6*12 dB(i.e., 72 dB) of boost. Since each additional equalizer stage 204compensates for an additional length (e.g., an additional 1000 feet) ofcable, selecting one of the outputs of these discrete equalizer stagesalone would results in too crude a resolution to accurately equalize acable of arbitrary length (e.g., a 3024 foot cable).

In accordance with an embodiment, to provide for less crude (i.e., morefine) resolution, a pair of selectors 212 ₁ and 212 ₂ (e.g., each ofwhich can be a multiplexor) and a weighted averager 214 (e.g., amulti-position fader, such as a 64 position fader) are used tointerpolate or otherwise combine the outputs of two adjacent equalizerstages 204. Stated an equivalent way, the weighted averager 214 can beused to combine the input and the output of the last active equalizerstage 203. For example, where the weighted averager is a 64 positionfader, and each equalizer stage 204 _(n) compensates for an additional1000 feet of cable, this weighted averager 214 allows fine tuning theequalization to 1/64 of 1000 feet (˜16 feet).

In the embodiment shown, each selector 212 includes six differentialinputs (In0-In5) and one differential output, which is selected, e.g.,by a three bit input. In accordance with an embodiment, each attenuationstage 202 can be capable of selecting between no attenuation and apredetermined attenuation level (e.g., −6 dB). More or less equalizerstages 203 can be used, in alternative embodiments of the presentinvention. If a different total number of equalizer stages 203 are used,then the selectors 212 may have more or less inputs. In other words,alternate implementations can have more equalizer stages with less boostper stage, or less stages with more boost per stage, with acorresponding change in the number of inputs to selectors 212.

Referring to FIG. 2B, each attenuation stage (e.g., 202 ₀ and 202 ₁ inFIG. 2B) can be implemented using a resistor ladder 216 and a pair of2-input/1-output multiplexors 218, or 2-input/1output weighted averagers218 (e.g., faders). Block 218 can selects between a non-attenuatedsignal path (not stepped down by the resistor ladder 216) and a steppeddown version of the signal (which has been stepped down by the resistorladder 216), or potentially variations in-between. For example, theoutput of block 218 can be adjustable to select either of its twoinputs, the mean of its two inputs, or any weighted average of its twoinputs, to within the resolution of the number of bits controlling it.In one embodiment 6 control bits are used, but more or less bits arepossible depending on the required resolution/flatness of the system.Still referring to FIG. 2B, in accordance with an embodiment, eachequalizer stage 204, which accepts a differential input, can beimplemented by a pair of operational amplifiers 220 configured asnon-inverting amplifiers, or a single differential operational amplifiercan be used. For example, equalizer stage 204 ₁ in FIG. 2B can beequivalently implemented using a differential amplifier or differentialop-amp in conjunction with resistors and capacitors that implement anequivalent response. The values of the resistors and capacitors within aspecific equalizer stage 204 should be optimized for the length of cablefor which that equalizer stage is to perform compensation. For example,referring back to FIG. 2A, the equalizer stage 204 ₁ can be optimizedfor a first 1000 feet of cable, the equalizer stage 204 ₂ can beoptimized for a second 1000 feet of cable, . . . and the equalizer stage204 ₆ can be optimized for a sixth 1000 feet of cable. The high levelsof required signal boost present a headroom issue. One solution to thisheadroom issue would be to attenuate the input signal 101, but the largeamount of required equalization boost presents a noise problem, sincethe noise of the input stage is multiplied by the amount of selectedboost which can exceed 60 dB with as much as another 12 dB of excessboost internal to the equalizer. To achieve the lowest reasonable inputstage noise, the input preferably should not be attenuated, but rathershould be amplified, before reaching the equalizer. Accordingly, itwould be preferable that the gain stage 202 ₀ not be present, or ifpresent, provide for unity gain. Thus, the high band equalizer 104should operate at the largest signal swing possible to minimize itscontribution to output noise. This low noise requirement is in conflictwith the potential overload created by the unused portion of the lastactive equalizer stages 204 (this phenomenon of overload is explained inmore detail below).

An alternate implementation could incorporate more equalizer stages,each stage with a more modest level of boost, proportionally reduced bythe ratio of increase in stages, e.g., using twice the number of stages,each stage with 6 dB less gain results in an equivalent equalization.These variants are impractical since they require more power and moreamplifiers and are thus more expensive to implement and maintain.Accordingly, it would be beneficial to address the overload issuewithout significantly reducing input stage gain (to keep noise low) andwithout enlarging the equalizer by implementing more boost stages withless boost per stage (to keep cost low).

Potential overload occurs under any conditions where there is more boostequalization in the signal chain than is required to equalize the signalto have a flat frequency response. The root cause of overload conditionscan be understood by examining the circuit in FIG. 2A, where sixequalizer stages 204 are shown. If all these stages are enabled (i.e.,kept in the signal chain) regardless of the cable length, then unless avery long cable is used, overload will occur. Consider the extreme caseof a zero-length cable. The output of the first equalizer stage 204 ₁will be boosted by 12 dB at 6 MHz; and the output of the last equalizerstage 204 ₆ will be boosted by 70 dB at 6 MHz. These levels of boostwill overload any practical amplifier. Now consider another examplewhere 4000 feet of cable are used. Now the output of the 4th equalizerstage 204 ₄ will be flat, but the 5^(th) stage 204 ₅ will have 12 dB ofexcess boost and the following 6^(th) stage 204 ₆ will have 24 dB ofexcess boost. To avoid overloads, all unnecessary equalizer stagesshould be disabled or at least not use to produce the compensated videosignal output. For an arbitrary cable length, equalizing its responseusing the simple string of equalizer stages (6 stages in the embodimentshown), can result in either excess equalization or in-adequateequalization. The only exceptions would be for cable lengths of exactly0 feet, 1000 feet, 2000 feet, 3000 feet, 4000 feet, 5000 feet, and 6000feet (again, assuming each gain stage 204 provided for equalization ofan additional 1000 feet).

The use of the selectors 212 ₁ and 212 ₂ in FIG. 2A essentially allowsfor the disabling of any unnecessary equalizer stages to avoid overload,while also providing for interpolation. For example, where the In2inputs of selectors 212 ₁ and 212 ₂ are selected, the output of theequalizer stage 203 ₂ (which is also the input to the equalizer stage203 ₃) is provided by the selector 212 ₁ to one differential input ofthe weighted averager 214, and the output of the equalizer state 203 ₃is provided by the other selector 212 ₂ to the other differential inputof the weighted averager 214. A multibit (e.g., 9 bit) control signal146 generated by compensation controller 118 or 119 can control theselectors 212 ₁ and 212 ₂ and the weighted averager 214, with the mostsignificant bits (e.g., 3 MSBs) performing course tuning by controllingthe selectors 212 ₁ and 212 ₂, and the least significant bits (e.g., 6LSBs) performing fine tuning by controlling the weighed averager 214. Inthis example: the equalizer stages 203 ₁, 203 ₂ and 203 ₃ can beconsidered enabled or active, since they affect the signal produced bythe weighted averager 214; and the equalizer stages 203 ₄, 203 ₅ and 203₆ can be considered disabled or inactive, since they do not affect thesignal produced by the weighted averager 214.

The weighted averager 214 interpolates between the outputs of the lasttwo active equalizer stages 203, or equivalently, between the input andoutput of the last active equalizer stage 203. Overload can occur,because to achieve a composite response that is flat (the compositeresponse is defined as the cable's response multiplied by theequalizer's response), there is typically some excess boost that is notbrought to the equalizer's output, but exists at the output of the lastactive equalizer stage. For a hypothetical example, if the cable were4016 feet, the correct equalization would be derived by selecting the4000 foot and 5000 foot outputs of the equalizer stages (equivalentlythe input and output of the 5000 foot equalizer stage), and theninterpolating between them by selecting the an appropriate weightedaverage (e.g., a lowest fader tap of a 64 position fader) whichcorresponds to an equalization of 4000 feet plus 16 feet. This resultsin the appropriate equalization, but it produces a side effect in thatthe signal at the 5000 foot output tap is 12 db larger at 5 MHz than thecorrectly equalized (“flat”) signal amplitude. Since signal swing(against a limited headroom) has already been maximized to reduce noise,this 12 dB of unwanted boost exceeds the allowable signal level by ˜4×.This can be solved in practice as a compromise by reducing the inputgain by 6 dB (using attenuation stage 202 ₀) and causing a 6 dbattenuation into the last active equalizer stage 203. This compromisemaintains reasonable signal-to-noise while avoiding an overload in thesignal chain. In other words, each equalizer stage 203 _(n) can have aselectable 0 dB/6 dB attenuator 202 _(n) at its input.

The attenuator 202 ₀ provides for pass-through for all cases where morethan the first equalizer stage 202 ₁ is active, and for attenuation(e.g., −6 dB) where only the first equalizer stage 202 ₁ is active.Putting the attenuator 202 ₀ in front of the first equalizer stage 203 ₁increases noise (e.g., by 6 dB), but this only increases noise for casesof short cable runs where the high frequency boost is limited to at most12 dB. The added noise is harmless in the context of such modest levelsof equalization boost.

While most equalizers are viewed in the frequency domain, they can alsobe viewed in the time domain. This involves the impulse response of boththe cable and the equalizer stage(s). In accordance with an embodiment,an algorithm takes the cable transfer function and determines the bestlocation, e.g., for a two pole/two zero equalizer stage that minimizesthe waveform distortion in the time domain. One could simply put a knownwaveform (a square pulse, for example) through 1000 feet of cable, andadjust the resistors and capacitors of an equalizer stage 204 tominimize the distortion. Alternatively, an algorithm can do thisautomatically and optimally. A further point of the algorithm is to usethe first stage to compensate a 2000 foot length of cable, and apply thealgorithm (or adjust the resistors and capacitors of a second stage) tominimize the distortion of a square pulse travelling through 2000 feetof cable and two equalizer stages 204. After the first stage 2041 hasbeen optimized for the 1st 1000 feet of cable, then the second stage2042 is optimized for the 2nd 1000 feet, etc. Other methods can besimilarly applied by viewing the frequency response error on a networkanalyzer. In any case, the next stage is always optimized by applyingthe previously determined stages to a longer length of cable along withthe next stage. This minimizes accumulated error and effectively createsa multi-pole/multi-zero equalizer (e.g., a 2 pole/2 zero equalizer for1000 feet of cable, a 4 pole/4 zero equalizer for 2000 feet of cable . .. or a 12 pole/12 zero equalizer for 6000 feet of cable), making itextremely accurate and robust.

The equalizer stages 204 ₁ to 204 ₆ are sequential, so their effect iscumulative. In accordance with an embodiment of the present invention,each equalizer stage 204 of the high band equalizer 104 has a transferfunction equal to the inverse of the transfer function of theincremental length of cable for which the equalizer stage is intended toperform high band frequency compensation. More specifically, eachsuccessive equalizer stage has a transfer function equal to the inverseof the transfer function of the sub-length of cable for which theequalizer stage 204 is intended to perform high band frequencycompensation, assuming all the earlier equalizer stage(s) are within thesignal path. For example: the equalizer stage 204 ₃ can have a transferfunction equal to the inverse of the transfer function of the third 1000feet of cable, for which the equalizer stage 204 ₃ is intended toperform high band frequency compensation, assuming that equalizer stages204 ₂ and 204 ₁ are within the signal path; the equalizer stage 204 ₂can have a transfer function equal to the inverse of the transferfunction of the second 1000 feet of cable, for which the equalizer stage204 ₂ is intended to perform high band frequency compensation, assumingthe equalizer stage 204 ₁ is within the signal path; and the equalizerstage 204 ₁ can have a transfer function equal to the inverse of thetransfer function of the first 1000 feet of cable, for which theequalizer stage 204 ₁ is intended to perform high band frequencycompensation.

In accordance with an embodiment, the attenuator 202 of the second tolast active equalizer stage 203 is the only attenuator that shouldprovide attenuation (e.g., −6 dB). In another embodiment, the attenuator202 of the last active equalizer stage 203 is the only attenuator thatshould provide attenuation (e.g., −6 dB), and an attenuator is added atthe output of the selector 212 ₁. Both of these embodiments will ensurethat the signals provided to the weighted averager 214 have the sameamplitude. For either embodiment, attenuator control logic 213 shown inFIG. 2B, can determine and control which attenuator 202 should provideattenuation based on the same course tune signal that controls theselectors 212 (e.g., by controlling the switches 214 in FIG. 2B).

Additional details of the low band equalizer 108, according to anembodiment of the present invention, will now be described withreference to FIG. 3. In accordance with an embodiment, the low bandequalizer 108 includes a unity gain buffer 302 and operational amplifier304 configured as a non-inverting amplifier with, e.g., a gain factor ofabout 2 (i.e., about 6 db). The outputs of the buffer 302 and theamplifier 304 are provided to a weighted averager 314 (e.g., amulti-position fader, such as a 64 position fader). The weightedaverager 314 and an adjustable capacitance C_adjust (e.g., a bank ofcapacitors connected in parallel that can be selectively switched intoand out of the circuit) are controlled by a low band controller 306which receives control signal 147 from the compensation controller 118or 119. The low band controller 306, based on the signal 147, can use alook-up table (e.g., a look up ROM, or the like) to select theappropriate capacitance and weighting appropriate for the length of thecable. As the cable gets longer, both the pole and zero have to movedown in frequency, which is accomplished by adjusting the capacitanceC_adjust. Adjustment of the weighted averager 314 also affects thelocation of the zero(s) and pole(s). The above mentioned look-up can bebased on all the bits, or just certain number of the most significantbits, of the signal 147. Additionally, all of the bits, or just certainnumber of the most significant bits, can be used to adjust the weightedaverager 314. An alternative implementation could use a fixed capacitorwith a variable resistor array. It is only required that the frequencyresponse is substantially similar to what is implemented by FIG. 3A. Anyalternate mode of implementation for low band equalizer 108 should besufficient.

Low frequency attenuation caused by the cable results in the sync-tipbeing tilted in the time domain. Adjustments made by the low bandequalizer 108 alter the tilt of the sync-tip of the H-sync pulse, andthus, the low band equalizer 108 can also be referred to as a tiltcontroller. When properly set, the sync-tip will have minimum tilt,i.e., have substantially zero slope.

The low band equalizer 108, because it is adjustable separate from thehigh band equalizer 104, enables the equalizer 100 (and 100′) to notonly compensate for various lengths of cable, but also various types ofcable that have a different high to low band balance. For example, CAT5cable and CAT6 cable have a different high to low band balance.

Additional details of the level sense circuit 142 and the level detectto digital circuit 144, both of the automatic compensation controller118, will now be described with reference to FIG. 4. The level sensecircuit 142 includes a set of comparators 402 (and 404) that comparesthe level of portions of the output compensated video signal 116 (inFIG. 1) to a set of reference voltages, including, blanking level, burstlevel and sync level reference voltages (and a monochrome referencevoltage), as can be appreciated from FIG. 4. If a compared portion ofthe video output signal 116 is above the indicated reference voltage(monochrome, blanking level, burst level or sync level), then thecorresponding comparator output will be high, otherwise it is low. Themonochrome level, blanking level, burst level and sync level referencevoltages can be their corresponding nominal values. For example, theblanking level can be nominally zero volts, the burst level can benominally −150 mV, and the Sync level can be nominally −300 mV. There isa slight difference in these levels between the Phase Alternation byLine (PAL) and National Television System Committee (NTSC) standards,but this is not material to the invention. In accordance with anembodiment, all of these levels may be shifted to some other range forconvenience, but the relationship remains.

The level detect to digital circuit 144 processes the outputs of thecomparators 402 by using sense counters 412 to count the number of clockcycles each comparator 402 is high, during a relevant timing pulse(provided to one of two inputs of AND gates 408 _(n)). For example, thesense counter 412 ₁ produces a count output indicative of the extentthat the breezeway portion of a line of the compensated video signal 116is greater than a nominal blanking level (e.g., 0V); the sense counter412 ₂ produces a count output indicative of the extent that the colorburst (if one exists) of a line of the compensated video signal 116 isgreater than a nominal burst level (e.g., −150 mV); the sense counter412 ₃ produces a count output indicative of the extent thatsubstantially the entire sync tip of a line of the compensated videosignal 116 is greater than a nominal sync level (e.g., −300 mV); and thesense counter 412 ₄ produces a count output indicative of the extentthat a beginning portion of the sync tip of a line of the compensatedvideo signal 116 is greater than a nominal sync level (e.g., −300 mV).The most significant bit (MSB) of each sense counter 412 will be either0 or 1, and is used to cause one of the error integrator counters 414 tocount up or down. The sense counters 412 are reset once per line ofvideo (i.e., per H-sync).

When the compensated video signal output 116 is properly compensated forthe length of the cable, the output of the error integrator counters 414will be substantially constant. In an embodiment, the error integeratorcounters 414 can be selectively locked or prevented from counting up anddown (e.g., be disabled), after proper compensation has been achieved,and it is determined that the length of cable is not changing. This canbe done by monitoring the output of one of the sense counters (e.g., 412₁), and determining that the length of cable is not changing if theoutputs of that sense counter just prior to being reset deviates lessthan a specified amount.

Generally, the control loops including the comparators 402, sensecounters 412 and error integrator counters 414 try to adjustequalization and gain so that each comparator 402 is high a specifiedpercent (e.g., 50%) of the time. The error integrator counters 414generate the actual control signals (146, 147 and 148) for the loops.The longer the cable being compensated for, the higher the value of thehigh band control signal 146 output by the error integrating counter 414₁, since the longer the cable the more high frequency compensationneeded. Accordingly, referring back to FIG. 2A, the longer the cable,the greater the MSBs of the signal 146, and thus the more equalizerstages 203 that will be activated.

The output of the comparator 404, is averaged by an averager 405, andthereafter compared by the comparator 406 to a monochrome/color decisionlevel reference, which is indicative of whether the video signal is amonochrome (e.g., black and white) video signal (which does not includea color burst) or a color video signal. The output of the comparator 406controls a switch S1, so that the output of the appropriate sensecounter 412 ₁ or 412 ₂ is provided to the input of the error integratorcounter 414 ₁. In this manner, the control loop for high bandequalization is appropriately calibrated for monochrome or color videosignal.

In accordance with an embodiment, if the sync pulse to digital timingcircuit 134 loses lock of the H-sync pulses (e.g., because a differentlength of cable was switched in, e.g., by a mux 506 shown in FIG. 5B),the various control loops discussed below can be reset to default levels(e.g., ratcheted back to zero), and the automatic compensation mechanismdescribed above can restart. For example, all the counters 412 and 414can be reset to zero.

FIGS. 5A & 5B are high level block diagrams of systems that include theequalizer 100 or 100′ of FIGS. 1A or 1B. Referring to FIG. 5A, a cablelink 504 (e.g., an unshielded twisted pair such a CAT5 cable) isconnected between the transmitter 502 and an equalizer 100 or 100′ of anembodiment of the present invention. The signal that is transmitted overthe cable link 504 can be, e.g., a NTSC video signal, or some other typeof video signal. The equalizer 100 or 100′ compensates for the high andlow frequency attenuation caused by the cable link 504 and outputs acompensated video signal 116, which is provided to a display monitor 510.

Referring to FIG. 5B, a multiplexor (mux) 506 can be used to selectamong different video sources 500 (e.g., different surveillancecameras), each associated with a transmitter 502, where each transmitter502 is connected to the mux 506 by a cable link 504, each of which canhave a different length. When the mux 506 selects a different videosource 500, the automatic compensation controller 118 of the equalizer100 automatically adjusts its compensation for frequency attenuationthat occurs during the transmission over one of the cable links 504. Inthe case of the equalizer 100′, manual adjustment of the compensationcan be performed when the mux 506 is used to select a different videosource 500.

The high level flow diagram of FIG. 6 will now be used to summarizevarious embodiments of the present invention for providing automaticcompensation for frequency attenuation of a video signal transmittedover a cable. Referring to FIG. 6, at step 602, frequency attenuationthat occurred during the transmission of the video signal over the cableis compensated for to thereby produce a compensated video signal. Atstep 604, one or more portions of the compensated video signal is/arecompared to one or more reference voltage levels. At step 606, there isautomatic adjusting of the compensating performed at step 602 based onresults of the comparing performed at step 604.

In accordance with specific embodiments, step 602 can includecompensating for high frequency attenuation caused by the cable andcompensating for low frequency attenuation caused by the cable. Step 602can also include fine tuning DC gain so that an average level of synctips, of horizontal sync pulses within the compensated video signal, issubstantially equal to a predetermined nominal level.

Each line of the compensated video signal produced at step 602 includesa horizontal sync portion, followed by a breezeway portion, followed acolor burst portion (if the video signal is color), followed by anactive video portion. Step 604 can include comparing the horizontal syncportion of the compensated video signal to a sync level referencevoltage, comparing the breezeway portion of the compensated video signalto a blanking level reference voltage, and comparing the color burstportion of the compensated video signal to the burst level referencevoltage (if the video signal is a color signal). Step 606 can includeautomatically adjusting the compensating performed at step 602 based onresults of the just mentioned comparisons.

Additional details of steps 602-606 are provided in the detaileddescription of the previously discussed FIGS. For example, specificembodiments for providing compensation for high frequency attenuationthat occurred during the transmission of a video signal over a cable aredescribed with reference to FIGS. 2A and 2B. This can include providinga plurality N of equalizer stages connected in series, wherein each ofthe N equalizer stages includes a differential input and a differentialoutput, and wherein each of the N equalizer stages is optimized for adifferent portion of the cable (where N is equal to or greater than 3).Additionally, there is a selection of which equalizer stages are activeand which equalizer stages are inactive, and a weighted average of thesignal input to and output from the last active equalizer stage isproduced.

In accordance with specific embodiments, providing the plurality N ofequalizer stages connected in series includes optimizing a 1^(st) one ofthe equalizer stages for a 1^(st) length of the cable (e.g., a first1000 feet of the cable), optimizing a 2^(nd) one of the equalizer stagesfor a 2^(nd) length of the cable (e.g., a second 1000 feet of thecable), . . . and optimizing an N^(th) one of the equalizer stages foran N^(th) length of the cable (e.g., an Nth 1000 feet of the cable).

In accordance with specific embodiments, providing the plurality N ofequalizer stages connected in series includes implementing a transferfunction for a 1^(st) one of the equalizer stages as substantially equalto an inverse of a transfer function of a 1^(st) length of the cable(e.g., a first 1000 feet of the cable), implementing a transfer functionfor a 2^(nd) one of the equalizer stages as substantially equal to aninverse of a transfer function of a 2^(nd) length of the cable (e.g., asecond 1000 feet of the cable), . . . and implementing a transferfunction for a N^(th) one of the equalizer stages as substantially equalto an inverse of a transfer function of a N^(th) length of the cable(e.g., a N^(th) 1000 feet of the cable).

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have often been arbitrarily defined herein for theconvenience of the description. Unless otherwise specified, alternateboundaries can be defined so long as the specified functions andrelationships thereof are appropriately performed. Any such alternateboundaries are thus within the scope and spirit of the claimedinvention.

The forgoing description is of the preferred embodiments of the presentinvention. These embodiments have been provided for the purposes ofillustration and description, but are not intended to be exhaustive orto limit the invention to the precise forms disclosed. Manymodifications and variations will be apparent to a practitioner skilledin the art. Embodiments were chosen and described in order to bestdescribe the principles of the invention and its practical application,thereby enabling others skilled in the art to understand the invention.Slight modifications and variations are believed to be within the spiritand scope of the present invention. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. A system to provide automatic compensation for frequency attenuationof a video signal transmitted over a cable, comprising: an equalizerthat receives a video signal that was transmitted over a cable, providescompensation for frequency attenuation that occurred during thetransmission over the cable, and outputs a compensated video signal; anda compensation controller that automatically adjusts the compensationprovided by the equalizer based on comparisons of one or more portionsof the compensated video signal to one or more reference voltage levels.2. The system of claim 1, wherein the equalizer includes: a high bandequalizer that compensates for high frequency attenuation caused by thecable; a low band equalizer that compensates for low frequencyattenuation caused by the cable; and a DC gain controller that finetunes DC gain of the equalizer so that an average level of sync tips, ofhorizontal sync pulses within the compensated video signal, issubstantially equal to a predetermined nominal level.
 3. The system ofclaim 2, wherein the compensation controller automatically adjusts thecompensation provided by the equalizer by automatically controlling thehigh band equalizer, the low band equalizer and the DC gain controller.4. The system of claim 3, wherein: each line of the compensated videosignal includes a horizontal sync portion, followed by a breezewayportion, followed a color burst portion if the video signal is color,followed by an active video portion; the reference voltage levelsinclude a sync level reference voltage, a blanking level referencevoltage and a burst level reference voltage; and the compensationcontroller includes a first comparator to compare the horizontal syncportion of the compensated video signal to the sync level referencevoltage; a second comparator to compare the breezeway portion of thecompensated video signal to the blanking level reference voltage; and athird comparator to compare the color burst portion of the compensatedvideo signal to the burst level reference voltage, if the video signalis color; and wherein the compensation controller is configured tocontrol the low band equalizer in dependence on an output from the firstcomparator; control the DC gain controller in dependence on an outputfrom the first comparator; control the high band equalizer in dependenceon an output from the second comparator, if the video signal is amonochrome signal; and control the high band equalizer in dependence onan output from the third comparator, if the video signal is a colorsignal.
 5. The system of claim 3, wherein: the each line of thecompensated video signal includes a horizontal sync portion followed bya breezeway portion, followed a color burst portion if the video signalis color, followed by an active video portion; and the compensationcontroller performs comparisons of the horizontal sync portion of thecompensated video signal to a sync level reference voltage, and controlsthe low band equalizer and the DC gain control based on results of thecomparisons.
 6. The system of claim 3, wherein the high band equalizercomprises: a plurality N of equalizer stages connected in series,wherein each of the N equalizer stages includes a differential input anda differential output, and wherein each of the N equalizer stages isoptimized for a different portion of the cable, where N is equal to orgreater than 3; a first selector having N inputs connected to the inputsof each of the N equalizer stages, and having an output; and a secondselector having N inputs connected to the outputs of each of the Nequalizer stages, and having an output; and a weighted averager havinginputs connected to the outputs of the first and second selectors, andhaving an output; wherein the first and second selectors are used toselect which equalizer stages are active and which equalizer stages areinactive; wherein the weighted averager produces at its output, aweighted average of the signal input to and output from the last activeequalizer stage; and wherein the compensation controller controls thefirst and second selectors and the weighted averager.
 7. The system ofclaim 6, wherein: a 1^(st) one of the equalizer stages is optimized fora 1^(st) length of the cable; a 2^(nd) one of the equalizer stages isoptimized for a 2^(nd) length of the cable; and an N^(th) one of theequalizer stages is optimized for an N^(th) length of the cable.
 8. Thesystem of claim 7, wherein: a 1^(st) one of the equalizer stages has atransfer function substantially equal to an inverse of a transferfunction of a 1^(st) length of the cable; a 2^(nd) one of the equalizerstages has a transfer function substantially equal to an inverse of atransfer function of a 2^(nd) length of the cable; and a N^(th) one ofthe equalizer stages has a transfer function substantially equal to aninverse of a transfer function of a N^(th) length of the cable.
 9. Thesystem of claim 6, wherein: a 1^(st)one of the equalizer stages has atransfer function substantially equal to an inverse of a transferfunction of a 1^(st) length of the cable; a 2^(nd) one of the equalizerstages has a transfer function substantially equal to an inverse of atransfer function of a 2^(nd) length of the cable; and a N^(th) one ofthe equalizer stages has a transfer function substantially equal to aninverse of a transfer function of a N^(th) length of the cable.
 10. Thesystem of claim 6, wherein the compensation controller uses outputs fromthe first comparator to automatically select: one of the N outputs ofthe first selector; one of the N outputs of the second selector; and aweighting performed by the weighted averager.
 11. A method for providingautomatic compensation for frequency attenuation of a video signaltransmitted over a cable, comprising: (a) compensating for frequencyattenuation that occurred during the transmission of the video signalover the cable to thereby produce a compensated video signal; (b)comparing one or more portions of the compensated video signal to one ormore reference voltage levels; and (c) automatically adjusting thecompensating performed at step (a) based on results of the comparingperformed at step (b).
 12. The method of claim 11, wherein step (a)include: (a.1) compensating for high frequency attenuation caused by thecable; (a.2) compensating for low frequency attenuation caused by thecable; and (a.3) fine tuning DC gain so that an average level of synctips, of horizontal sync pulses within the compensated video signal, issubstantially equal to a predetermined nominal level.
 13. The method ofclaim 12, wherein step (c) includes: (c.1) automatically adjusting thecompensating for high frequency attenuation performed at step (a.1);(c.2) automatically adjusting the compensating for low frequencyattenuation performed at step (a.2); and (c.3) automatically adjustingthe fine tuning of DC gain performed at step (a.3).
 14. The method ofclaim 13, wherein: each line of the compensated video signal produced atstep (a) includes a horizontal sync portion, followed by a breezewayportion, followed a color burst portion if the video signal is color,followed by an active video portion, and the comparing performed at step(b) includes (b.1) comparing the horizontal sync portion of thecompensated video signal to a sync level reference voltage; (b.2)comparing the breezeway portion of the compensated video signal to ablanking level reference voltage; and (b.3) comparing the color burstportion of the compensated video signal to the burst level referencevoltage, if the video signal is a color signal; and the automaticallyadjusting performed at step (c) includes (c.1) automatically adjustingthe compensating for high frequency attenuation performed at step (a.1)based on results of the comparing performed at step (b.3), if the videosignal is a color signal; (c.2) automatically adjusting the compensatingfor high frequency attenuation performed at step (a.1) based on resultsof the comparing performed at step (b.2), if the video signal is amonochrome signal; (c.3) automatically adjusting the compensating forlow frequency attenuation performed at step (a.2) based on results ofthe comparing performed at step (b.1); and (c.4) automatically adjustingthe fine tuning of DC gain performed at step (a.3) based on results ofthe comparing performed at step (b.1).
 15. The method of claim 11,wherein: each line of the compensated video signal produced at step (a)includes a horizontal sync portion followed by a breezeway portion,followed a color burst portion if the video signal is color, followed byan active video portion; step (b) includes performing comparisons of thehorizontal sync portion of the compensated video signal to a sync levelreference voltage; step (c) includes automatically adjusting DC gainbased on results of the comparisons.
 16. A method for providingautomatic compensation for frequency attenuation of a video signaltransmitted over a cable, comprising: producing a compensated videosignal by compensating for high frequency attenuation caused by thecable, compensating for low frequency attenuation caused by the cable,and fine tuning DC gain so that an average level of sync tips, ofhorizontal sync pulses within the compensated video signal, issubstantially equal to a predetermined nominal level; comparing ahorizontal sync portion of the compensated video signal to a sync levelreference voltage, a breezeway portion of the compensated video signalto a blanking level reference voltage, and a color burst portion of thecompensated video signal to a burst level reference voltage; andautomatically adjusting the compensating for high frequency attenuation,the compensating for low frequency attenuation and the fine tuning of DCgain based on results of the comparing.
 17. A high band equalizer thatprovides compensation for high frequency attenuation that occurredduring the transmission of a video signal over the cable, comprising: aplurality N of equalizer stages connected in series, wherein each of theN equalizer stages includes a differential input and a differentialoutput, and wherein each of the N equalizer stages is optimized for adifferent portion of the cable, where N is equal to or greater than 3; afirst selector having N inputs connected to the inputs of each of the Nequalizer stages, and having an output; and a second selector having Ninputs connected to the outputs of each of the N equalizer stages, andhaving an output; and a weighted averager having inputs connected to theoutputs of the first and second selectors, and having an output; whereinthe first and second selectors are used to select which equalizer stagesare active and which equalizer stages are inactive; and wherein theweighted averager produces at its output, a weighted average of thesignal input to and output from the last active equalizer stage.
 18. Thehigh band equalizer of claim 17, wherein a compensation controllercontrols the first and second selectors and the weighted averager. 19.The high band equalizer of claim 17, wherein: a 1^(st) one of theequalizer stages is optimized for a 1^(st) length of the cable; a 2^(nd)one of the equalizer stages is optimized for a 2^(nd) length of thecable; and an N^(th) one of the equalizer stages is optimized for anN^(th) length of the cable.
 20. The high band equalizer of claim 17,wherein: a 1^(st) one of the equalizer stages has a transfer functionsubstantially equal to an inverse of a transfer function of a 1^(st)length of the cable; a 2^(nd) one of the equalizer stages has a transferfunction substantially equal to an inverse of a transfer function of a2^(nd) length of the cable; and a N^(th) one of the equalizer stages hasa transfer function substantially equal to an inverse of a transferfunction of a N^(th) length of the cable.
 21. A method for providingcompensation for high frequency attenuation that occurred during thetransmission of a video signal over the cable, comprising: (a) providinga plurality N of equalizer stages connected in series, wherein each ofthe N equalizer stages includes a differential input and a differentialoutput, and wherein each of the N equalizer stages is optimized for adifferent portion of the cable, where N is equal to or greater than 3;(b) selecting which equalizer stages are active and which equalizerstages are inactive; and (c) producing a weighted average of the signalinput to and output from the last active equalizer stage.
 22. The methodof claim 21, wherein step (a) comprises: (a.1) optimizing a 1^(st) oneof the equalizer stages for a 1^(st) length of the cable; (a.2)optimizing a 2^(nd) one of the equalizer stages for a 2^(nd) length ofthe cable; and (a.3) optimizing an N^(th) one of the equalizer stagesfor an N^(th) length of the cable.
 23. The method of claim 21, whereinstep (a) comprises: (a.1) implementing a transfer function for a 1^(st)one of the equalizer stages as substantially equal to an inverse of atransfer function of a 1^(st) length of the cable; (a.2) implementing atransfer function for a 2^(nd) one of the equalizer stages assubstantially equal to an inverse of a transfer function of a 2^(nd)length of the cable; and (a.3) implementing a transfer function for aN^(th) one of the equalizer stages as substantially equal to an inverseof a transfer function of a N^(th) length of the cable.
 24. The systemof claim 1, wherein: the equalizer includes a low band equalizer thatcompensates for low frequency attenuation caused by the cable; and thecompensation controller automatically adjusts the compensation providedby the low equalizer band equalizer stage based on comparisons between ahorizontal sync portion of the compensated video signal and a sync levelreference voltage.
 25. The system of claim 24, wherein automaticadjustments by the compensation controller, of the compensation providedby the low equalizer band equalizer stage, adjusts tilt of thehorizontal sync portion of the compensated video signal.
 26. The systemof claim 24, wherein automatic adjustments by the compensationcontroller, of the compensation provided by the low equalizer bandequalizer stage, minimizes tilt of the horizontal sync portion of thecompensated video signal.
 27. The method of claim 11, wherein: step (a)include compensating for low frequency attenuation caused by the cable;step (b) includes comparing a horizontal sync portion of the compensatedvideo signal to a sync level reference voltage; and step (c) includesautomatically adjusting the compensating for low frequency attenuationperformed at step (a) based on a result of the comparing the horizontalsync portion of the compensated video signal to the sync level referencevoltage performed at step (b).
 28. The method of claim 27, wherein theautomatically adjusting the compensating for low frequency attenuationadjusts tilt of the horizontal sync portion of the compensated videosignal.
 29. The method of claim 27, wherein the automatically adjustingthe compensating for low frequency attenuation minimizes tilt of thehorizontal sync portion of the compensated video signal.